Heat fluxes and thermal stability in the mixolimnion of a tropical heliothermal meromictic crater lake

An 11-month high-resolution dataset from a rare, hypersaline crater lake on Mexico's Isla Isabel reveals its intense heliothermal regime is not a permanent state but a distinct seasonal phenomenon. The development of a subsurface temperature maximum exceeding 47 °C is driven by ectogenic meromixis, where a massive influx of clear freshwater during the rainy season forms a low-density surface cap, suppressing vertical mixing and trapping solar radiation in the denser, saltier layers below. Salinity is therefore the fundamental control on the lake's physical dynamics, creating the precondition for massive subsurface energy storage. This study provides the first definitive characterization of a complete annual cycle in a tropical heliothermal lake, establishing it as a critical model system for understanding the physical stability and response of these rare ecosystems to seasonal climatic forcing.